linux cgroup: also fix controllers()

This fix was found by Claude Code, but I manually reviewed this change
and removed extraneous changes made by the AI tool.

Co-authored-by: moderation <michael@sooper.org>

src/os/cgroup.zig

@@ -185,9 +185,7 @@ pub fn controllers(alloc: Allocator, cgroup: []const u8) ![]const u8 {
 
     // Read it all into memory -- we don't expect this file to ever
     // be that large.
-    var reader_buf: [4096]u8 = undefined;
-    var reader = file.reader(&reader_buf);
-    const contents = try reader.interface.readAlloc(
+    const contents = try file.readToEndAlloc(
         alloc,
         1 * 1024 * 1024, // 1MB
     );